Test generic function pointer generator

test.hpp

/** 
##### Header file test.hpp #####
*/

#ifndef __TEST_HPP__
#define __TEST_HPP__

#include <complex>
#include <iostream>

// is_complex type checker
template <class T>
struct is_complex : public std::false_type
{
};

// is_complex type checker
template <class T>
struct is_complex<std::complex<T>> : public std::true_type
{
};

// actual function, just prints the value val
template <typename T>
void func(T val)
{
    if (is_complex<T>::value)
        std::cout << "(" << val.real() << " + "
                  << "j" << val.imag() << ")" << std::endl;
    else
        std::cout << val << std::endl;
}

// func() caller with an opaque parameter
template <typename T>
void func_opaque(void *val)
{
    T *v;
    v = reinterpret_cast<T *>(val);
    func(*v);
}

// function factory
template <typename fnT, typename T>
struct Factory
{
    fnT get()
    {
        fnT f = func_opaque<T>;
        return f;
    }
};

// function pointer to the func_opaque
typedef void (*fptr)(void *);

#endif  // ########## End of header test.hpp


/** 
########## test.cpp ###########
###### File for testing 1 #####
*/
#include "test.hpp"
#include <complex>

int main(void)
{
    fptr f = Factory<fptr, std::complex<double>>{}.get();
    std::complex<double> z = 1.0 + 0.5i;
    f(&z);
    return 0;
}

// compile: icpx -I. test.cpp
// run: ./a.out
// output: (1 + j0.5)


/** 
########## test.cpp ###########
###### File for testing 2 #####
*/
#include "test.hpp"
#include <complex>

int main(void)
{
    fptr f1 = Factory<fptr, double>{}.get();
    double d = 7.0;
    f1(&d);
    return 0;
}

/** 
In file included from test.cpp:1:
./test.hpp:21:32: error: member reference base type 'double' is not a structure or union
        std::cout << "(" << val.real() << " + "
                            ~~~^~~~~
./test.hpp:32:5: note: in instantiation of function template specialization 'func<double>' requested here
    func(*v);
    ^
./test.hpp:40:17: note: in instantiation of function template specialization 'func_opaque<double>' requested here
        fnT f = func_opaque<T>;
                ^
test.cpp:10:39: note: in instantiation of member function 'Factory<void (*)(void *), double>::get' requested here
    fptr f1 = Factory<fptr, double>{}.get();
                                      ^
1 error generated.
*/